It is possible to apply the principles of the exemplary embodiments of present invention to the scope of MIMO spatial multiplexing or smart antenna system with spatial diversity, adaptive beamforming and advanced Tx signal monitoring capability as provided herein. Thus, it may be beneficial to provide a brief overview of these systems so as to better understand for which subject matters patent protection is sought.
With advances in digital signal processing integrated circuits, multiple-antenna multiple-output (MIMO) systems have emerged in which mobile terminals incorporate smart antenna systems comprising multiple transmit antennas and multiple receive antennas and apply spatial diversity techniques to transmit multiple versions of a signal. These spatial diversity techniques provide effective ways to combat multipath fading and to significantly mitigate co-channel interference in a wireless communication system. One important concept in spatial diversity is that the propagation of multiple versions of an RF signal from different antennas (“spatial stream”) may significantly reduce the probability of flat fading at the receiving mobile terminal since not all of the transmitted signals would have the same dead zone.
Recently, MIMO spatial multiplexing systems with a first antenna array consisting of multiple transmit antennas on a wireless transmitter side and a second antenna array consisting of multiple receive antennas on a wireless receiver side are becoming increasingly popular, which is owing to the fact that these systems provide an increased data rate without the need for an increase in signal bandwidth since the applied transmit antennas are able to transmit independent data streams. These in dependent data streams, however, can interfere with each other such that a complex channel equalizer is required at the wireless receiver side in order to separate RF signals received via different signal propagation paths. Furthermore, antenna diversity gain is reduced due to the correlation of the channel impulse responses of the wireless signal propagation paths between the transmit antennas of the antenna array on the wireless transmitter side and the receive antennas of the antenna array on the wireless receiver side. Using a compact space diversity receiver in handheld phones and portable terminals ensures that received RF signals will be at least partially correlated due to the compact nature of the antenna array in which antenna elements are typically spaced by a distance of a fraction of the RF signal wavelength.
Conventional smart adaptive antenna systems can combine multiple antenna elements with a signal processing capability to optimize the pattern of transmitted signal radiation and/or reception in response to the communications medium environment. The process of optimizing the pattern of radiation can be referred to as “adaptive beamforming”, which may utilize linear array mathematical operations to increase the average signal-to-noise ratio (SNR) by focusing energy in desired directions. In brief, adaptive beamforming is a technique in which an array of antennas is exploited to achieve maximum reception in a specified direction by estimating the signal arrival from a desired direction (in the presence of noise) while signals of the same frequency from other directions are rejected. At the same time, in transmit direction, the smart antenna base station generates either a group of fixed beams or a group of dynamically directionally steerable beams and, based on the location, i.e. the direction of the mobile unit, sends the downlink signal to that particular mobile unit only to the fixed beam which covers the location of the mobile unit best or adjusts the direction of a steerable beam to best cover said mobile unit. This is achieved by varying the weights of each of the sensors (antennas) used in the array. In adaptive beamforming, the optimum weights are iteratively computed using complex algorithms based upon different criteria.
MIMO systems with beamforming capability can facilitate simultaneous transmission of multiple signals occupying a shared frequency band, similar to what may be achieved in code division multiple access (CDMA) systems. For example, the multiplicative scaling of signals prior to transmission, and a similar multiplicative scaling of signals after reception, may facilitate a specific antenna at a receiving mobile terminal to receive a signal which had been transmitted by a specific antenna at the transmitting mobile terminal to the exclusion of signals which had been transmitted from other antennas. However, MIMO systems may not require the frequency spreading techniques used in CDMA transmission systems. Thus, MIMO systems may make more efficient utilization of frequency spectrum.